Our invention broadly relates to electrical and electronic protection systems that monitor and sense incoming electrical currents. More particularly, the present invention relates to digital control or sensing devices that monitor current in polyphase electrical systems and control downstream electrical loads such as motors and the like.
As will be appreciated by those skilled in the art, the need to provide protective circuits that prevent damage to relatively sensitive electrical components is paramount. The earliest type of protection devices was the well known lead fuses. Fuses continue to serve many important roles in modern electronics. However, more advanced means of protection that employ narrower safety margins are called for in modern applications. Additionally, "smart" devices that would provide diagnostic readouts and/or store operating histories that could later be gleaned from the device to determine the existence of fault conditions or to aid in preventive maintenance would be quite useful.
The prior art is replete with devices both analog and digital that are intended to measure current flow based upon variable conditions. Among the prior art relevant to the present disclosure, Wilson, U.S. Pat. No. 4,229,694, issued Oct. 21, 1980, discloses a power angle relay intended to measure the power angle of a synchronous alternator. Segger, U.S. Pat. No. 4,099,103, issued Jul. 4, 1978, discloses a device for measuring the angular position of a rotor of a synchronous motor.
Emanuel, U.S. Pat. No. 4,348,892, issued Sep. 14, 1982, discloses an apparatus for determining the power angle in Alternating Current machines. Stacey, U.S. Pat. No. 4,669,024, issued May 26, 1987, discloses a multiphase frequency selective phase locked loop with multiphase sinusoidal and digital outputs. Hamby, U.S. Pat. No. 4,104,570, issued Aug. 1, 1978, discloses a digital control system for electric motors. It teaches a technique to time control events over the entire range of motor operation. Hartman, U.S. Pat. No. 4,227,137, issued Oct. 7, 1980, discloses a digital tachometer and slip signal motor control. Stacey, U.S. Pat. No. 4,991,429, issued Feb. 12, 1991, teaches a torque angle and peak current detector for synchronous motors.
Also pertinent to our invention is U.S. Pat. No. 4,979,069, issued Dec. 18, 1990, to Elmer Simpson, a co-inventor of the present invention. A motor fault detector with optical isolation is disclosed by Mr. Simpson in his prior patent. The device is a series of ammeters connected to each phase of an electric motor. Each ammeter is intended to selectively indicate the current flow during start up and run of the motor. It further employs optical isolators responsive to the ammeters and a relay responsive to the optical isolators. When the needles of the ammeters block the optical output from the optical isolators a fault condition is determined in the motor and an appropriate response is initiated. Additional background information relating to the importance of motor winding current monitoring is seen in the background section of U.S. Pat. No. 4,979,069.
Damage to electrical circuits is usually caused by excess current. While the causes of increased current are numerous almost all are hazardous to relatively delicate electrical components. In addition to the above mentioned fuses other types of devices have been used to detect and overcome excessive current loads. Among these devices are phase loss detectors, motor starters utilizing overload heater coils and contacts, bimetallic contacts, and coil-solder heater tubes. A shortcoming of such devices is their general failure to respond quickly enough when a dangerous condition is present to prevent damage to relatively sensitive components.
Therefore, it is desirous to devise a protection device that will be able to detect minor fluctuations in current flow and to evaluate the causes thereof. Such a device would allow a logical series of conditions to dictate whether a shutdown of the electrical circuit was necessary or whether some other action should be taken. Fuses are generally designed to "blow out" after ten seconds at two hundred to six hundred percent of an overload current.
The present device will allow the evaluation to be made of the current over a period of milliseconds rather than seconds to determine whether the circuit should be shutdown or other action taken. Furthermore, the ideal device would facilitate tracking of the current thereby allowing a determination of whether any fault conditions were developing over a period of time. Such information could be stored in temporary memory associated with the before-mentioned logic circuits, not unlike the flight recorder of modern aircraft. Hence, the device would necessarily employ an interface with conventional computer equipment to download the data it has acquired. Computer circuitry can store the data for various circuits to determine over a period of time any fault conditions which may be developing or to otherwise evaluate circuit performance. Finally, such a device would need to operate in a stand alone capacity.
An ideal device would be capable of simple and effective computer programming, thereby allowing maintenance personnel to manually reconfigure the device as might be required from time to time. Ideally the apparatus must instantly stop starting current when the load instantaneous current is too high. Moreover, such a device must readily react to unbalanced phases, which are the principal cause of motor failure.